Since many body structures are composed of substantially the same tissue substances, it is generally difficult to differentiate such parts in an x-ray image. To permit easier viewing of selected body structures, a radio-opaque dye or contrast agent is frequently applied to such structure before the x-ray is taken. For example, conventional coronary anteriograms are generally acquired by inserting a catheter into a patient's peripheral artery and feeding the catheter through the arterial system to the heart. The physician then positions the tip of the catheter, under the guidance of an x-ray fluoroscope image, in the artery of interest, and a bolus of undiluted iodine contrast agent is injected through the catheter into the artery. The resulting high-quality x-ray image of the artery reveals the geometry of the arterial wall and allows, among other things, the identification of regions where blood flow may be reduced because of a blockage.
The complicated coronary angiography procedure described above is necessary because of the low sensitivity of conventional x-ray imaging systems to iodine-based contrast agents. This procedure is, however, both costly and hazardous. The catheterization procedure may cause myocardial infarction or stroke and the high concentration of the iodine dye may cause kidney problems including renal failure. The catheterization requires that the procedure be performed in an operating room with the patient normally being in the hospital for two days. As a result, the cost of the procedure can be several thousand dollars.
Because of both the costs and risks indicated above, coronary angiograhy is not normally utilized as a diagnostic or screening tool, but is reserved only for symptomatic patients where coronary artery disease may already have reached an advanced stage. Since coronary artery disease and its complications are the most common cause of death and disability in the middle-aged and elderly American population, a need exists for an improved method and apparatus for performing this procedure so as to reduce both the costs and the risks involved, thus permitting the procedure to be more extensively used. In particular, the improved procedure should permit the costs and risks to be reduced to the point where coronary angiography could be utilized as a routine diagnostic and screening tool.
One technique which has been employed for reducing the amount of iodine required to do coronary angiography is energy-subtraction imaging which takes advantage of the abrupt K-edge discontinuity in the iodine x-ray absorption spectrum to increase the sensitivity of the imaging system to body structures which have been iodinated. The K-edge discontinuity results from the absorption of an x-ray photon by an electron in the K.sub..alpha. shell of the iodine atom resulting in ionization and permitting an electron from the L shell to fall into the K.sub..alpha. shell vacancy. Because of this discontinuity, if images are taken at energy levels slightly above and slightly below the K-edge within a short time period compared to a heart cycle, and these images are logarithmically subtracted, the contrast produced by noniodinated structures cancel, whereas the contrast due to the iodinated structures remains due to the difference in the iodine absorption coefficient of the two images. Since this technique permits the removal of shadow images caused by bone, tissue and the like, it theoretically permits high-quality arteriograms to be produced using low concentrations of iodine-contrast agent. Reductions in required concentration of the iodine-contrast agent by a factor of approximately 25 may be possible utilizing this technique, permitting the iodine-contrast agent to be injected into a vein and flow to the desired heart area. This high reduction in concentration factor is necessary in order to employ the contrast agent in this way, because of the substantial dilution of the contrast agent as it flows from the injection site to the site where the image is to be taken. It is also desirable to reduce the amount of iodine contrast agent initially required. This procedure thus has the potential for substantially reducing risk factors in the angiograhy procedure by eliminating the need for a catheter and by reducing the iodine concentration level required. It would also reduce the cost of the procedure by making it possible for the procedure to be performed on an outpatient basis. The combination of improved safety and reduced cost might make the procedure available for screening use in nonsymptomatic cases, and to monitor progress after various coronary treatment procedures.
However, conventional broad-band x-ray sources with energy filtering or dual electron beam energies have been unable to provide the required photon fluences for energy subtraction imaging of the human coronary arteries. Narrow band x-ray beams from synchrotron sources with monochromators have produced high quality images. However, the costs associated with even a small synchrotron facility are currently about $30,000,000, a cost which is prohibitively high for most clinical applications. A need therefore exists for an improved apparatus and procedure for performing dual energy or energy-substraction medical imaging, and in particular energy subtraction coronary angiography, which permits this procedure to be performed at a cost which is consistent with that of comparable medical equipment.